Recently, optical recording media such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) are rapidly spreading as information recording media. The optical recording media are generally standardized to have an outer diameter of 120 mm and a thickness of 1.2 mm. The DVD uses a laser beam having a shorter wavelength than that for the CD as irradiation light. In addition, a numerical aperture of a lens for the irradiation light is set larger than that of the CD. As a result, the DVD is capable of recording and reproducing a larger amount of information at a higher density than the CD.
On the other hand, information recording and reproduction accuracy is more likely to lower as the wavelength of irradiation light becomes shorter and the numerical aperture of a lens becomes larger because coma aberration occurs due to inclination (warp) of a disc. Thus, the DVD includes a light transmitting layer having a half thickness of that of the CD, that is, 0.6 mm so as to ensure a margin for the inclination (warp) of the disc to keep the information recording and reproduction accuracy.
Since the light transmitting layer at a thickness of 0.6 mm alone does not offer sufficient stiffness and strength, the DVD has such a structure that two substrates, each having a thickness of 0.6 mm, are bonded to each other so that the information recording face faces inside. As a result, the DVD has a thickness of 1.2 mm, which is equal to that of the CD, to ensure the same stiffness and strength as those of the CD.
Moreover, a center hole (for example, at φ15 mm for CDs and DVDs) is generally formed in the optical recording medium so as to be used for positioning or the like in a recording and reproduction device or the like. The center hole is generally formed simultaneously with a molding step of molding a substrate in a disc-like shape.
In order to realize the recording of a larger amount of information at a higher density, there is a request for further reduction of the wavelength of irradiation light and further increase of the numerical aperture of a lens. In response to the request, an optical recording medium including a light transmitting layer at a further reduced thickness is needed. Therefore, such an optical recording medium that a light transmitting layer thinner than a substrate serving as a constructional member is formed on an information recording face of the substrate to achieve a total thickness of 1.2 mm is being developed. In order to standardize the specifications, it has been suggested to use a blue-violet laser beam having a wavelength of 405 nm as irradiation light and a numerical aperture of 0.85 and correspondingly to set a thickness of the light transmitting layer to 0.1 mm.
FIG. 18 is a perspective view showing a structure of an optical recording medium including such a thin light transmitting layer formed thereon.
An optical recording medium 100 is of single-sided type capable of recording information only on one side; it has such a structure that a light transmitting layer 104 thinner than a substrate 102 is formed on an information recording face 102A of the substrate 102.
The substrate 102 having a diameter of 120 mm and a thickness of 1.1 mm is generally formed by injection molding excellent in mass productivity. More specifically, after a resin such as polycarbonate is injected between a pair of molds, it is cooled and kept at a predetermined temperature. Then, it is formed in a disc-like shape.
The light transmitting layer 104 has a thickness of 0.1 mm, and is formed on the information recording face 102A of the substrate 102 by spin coating or the like. Specifically, an ultraviolet curable or electron beam curable, light transmitting resin is supplied to the vicinity of the center of the information recording face 102A. Then, the substrate 102 is rotated so as to force the supplied resin outward in radial direction by centrifugal force to allow it to flow. In this manner, the resin is spread over the entire surface of the information recording face 102A. After the spread, the resin is irradiated with an ultraviolet ray, an electron beam or the like so as to be cured, thereby completing a light transmitting layer formation step. In the case of dual-sided type capable of recording information on both sides of a substrate, a thickness of the substrate is set to 1.0 mm and a light transmitting layer at a thickness of 0.1 mm is laid on each of the faces of the substrate. Alternatively, two substrates, each having a thickness of 0.5 mm and including a light transmitting layer at a thickness of 0.1 mm deposited thereon, may be prepared and bonded to each other.
However, if the spin coating is applied to the substrate having a center hole formed therein, a thickness of the spread resin is likely to be nonuniform, sometimes resulting in difficulty in information recording and reproduction at a high density.
Although the reason for a nonuniform thickness is not exactly known, it is generally believed as follows. When a resin is supplied to the surroundings of a center hole, centrifugal force is immediately exerted on the resin by the rotation of a substrate. As a result, the resin is spread outward from the position of supply over the entire substrate while flowing outward in a radial direction from the position of supply. Meanwhile, the centrifugal force is still exerted. Therefore, a thickness of an inner part and that of an outer part do not become equal to each other, providing such a thickness profile that a thickness becomes smaller inward and becomes larger outward. It is conceivable to additionally supply a resin so as to compensate for the reduction in thickness. However, since highly accurate control in accordance with a variation in thickness is required, such control is difficult in practice.
On the other hand, with the use of a substrate without a center hole, a resin can be supplied to the center of the substrate or supplied radially inward as compared with the case of a substrate with a center hole. Therefore, the centrifugal force can be prevented from being immediately exerted on the supplied resin. In this case, since the center of the substrate serves as a resin reservoir to additionally supply the resin onto the information recording face in a continuous manner, the resin can be spread at a uniform thickness. In this case, after the spread resin is cured, a center hole is formed by punching the light transmitting layer 104 and the substrate 102 by a punching tool 106 as shown in FIG. 19. The reference numeral 102A in the drawing denotes a circular concave portion formed on a face of the substrate 102, which is opposite to the information recording face 102A, so as to facilitate the punching.
However, since the light transmitting layer 104 is extremely thin at a thickness of about 0.1 mm, there arises a problem in that a burr is generated on its inner periphery as shown in FIG. 20 or the inner periphery is stripped away from the information recording face 102A in some cases when the light transmitting layer is punched by the punching tool 106. Moreover, even if no burr or stripping occurs in a manufacturing step, a finger or the like touches the inner periphery of the light transmitting layer when the finger or the like is inserted into the center hole during the use, sometimes resulting in stripping of the light transmitting layer.